1. Field of the Invention
The present invention generally relates to an image processing apparatus and an image processing method, and more particularly to an image processing apparatus and an image processing method that encode image data by performing frequency transformation, for example, in accordance with JPEG (Joint Photographic Experts Group) 2000 standard.
2. Description of the Related Art
The JPEG2000 standard is well-known by those skilled in the art as one of image compression coding methods suitable for a high-definition image (“Next-Generation Image Coding Method JPEG2000”, Yasuyuki Nomizu, 2001, Triceps).
In image processing based on the JPEG2000 format, wavelet coefficients are obtained through two-dimensional discrete wavelet transformation. Then, the wavelet coefficients are decomposed into a plurality of bit-planes, and the least significant bit-plane data are discarded in a unit of subband. In this fashion, it is possible to adjust (decrease) the size of coded data that result form entropy coding on the wavelet coefficients.
According to the conventional JPEG2000 based image processing, however, when wavelet coefficients are entropy encoded, the data cannot be necessarily compressed at a constant data size. Accordingly, it is necessary to repeat the following process until the resulting coded data reaches to a desired size. Namely, the least significant bit-plane data of the wavelet coefficients are discarded, the remaining data of the wavelet coefficients are entropy coded, and the size of the resulting coded data is checked again. In general, it takes a large computation time to perform entropy coding because of a large amount of processed data. Thus, the above-mentioned conventional image processing method requires a large time to decrease (compress) coded data to a desired size.